In the past, there has been known an internal combustion engine which provides an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controls the amount of fuel fed to a combustion chamber of the internal combustion engine based on the output current of this air-fuel ratio sensor. The amount of fuel is controlled so that the air-fuel ratio of the air-fuel mixture burned in the combustion chamber becomes a target air-fuel ratio (for example, stoichiometric air-fuel ratio).
As one example of an air-fuel ratio sensor, there is known an air-fuel ratio sensor which linearly changes in output current (proportionally) with respect to an exhaust air-fuel ratio (for example, PTL 1). The output current becomes larger the higher the exhaust air-fuel ratio (the leaner). For this reason, the exhaust air-fuel ratio can be estimated by detecting the output current of the air-fuel ratio sensor.
In this regard, in an internal combustion engine, air-fuel mixture leaks out from a clearance between a piston and a cylinder block to the inside of a crankcase, that is, “blowby gas” is generated. If the blowby gas remains inside the crankcase, it will cause deterioration of the engine oil, corrosion of metal, air pollution, etc. Therefore, an internal combustion engine is provided with a blowby gas passage connecting the crankcase and the intake passage. The blowby gas passes through the blowby gas passage to be returned to the intake passage and is burned together with the new air-fuel mixture.
Further, in a cylinder injection type internal combustion engine directly injecting fuel into a combustion chamber, the distance between an injection port of a fuel injector and a cylinder wall surface is extremely short, and therefore the injected fuel directly strikes the cylinder wall surface. At the time of cold startup, the fuel deposited at the cylinder wall does not easily vaporize, and therefore it leaks out from the clearance between the piston and cylinder into the crankcase and is mixed with the engine oil. In other words, the engine oil inside the crankcase is diluted by the liquid phase fuel, that is, “oil dilution” occurs. On the other hand, after the internal combustion engine is warmed up, the temperature of the engine oil also rises, and therefore the fuel content in the engine oil vaporizes. Therefore, at the time of cold startup, if the internal combustion engine is warmed up while the amount of fuel contained in engine oil is small, the oil dilution rate will not increase much at all. Note that, the “oil dilution rate” is the value of the amount of fuel mixed in the engine oil divided by the amount of the engine oil.
However, if an operating state where the internal combustion engine is started at a low temperature and is stopped in a shorter time than the time by which the internal combustion engine is warmed up, a so-called “short trip”, is repeated, the amount of fuel content in the engine oil will increase. The oil dilution rate also increases. After that, if the internal combustion engine is warmed up, the large amount of fuel in the engine oil will vaporize, and therefore the fuel content in the blowby gas will increase. As a result, blowby gas containing a large amount of fuel will pass through the blowby gas passage and flow into the intake passage. For this reason, even if the amount of fuel injected from a fuel injector is controlled so that the air-fuel ratio of the air-fuel mixture becomes the target air-fuel ratio, a large amount of fuel is fed from the blowby gas passage, and therefore the air-fuel ratio deviates to the rich side with respect to the target air-fuel ratio. This sometimes causes obstructions to the various types of control of the air-fuel ratio such as air-fuel ratio feedback processing, and in turn causes deterioration of the driveability or exhaust emissions.
Therefore, in the control system of an internal combustion engine described in PTL 2, if oil dilution occurs, updating of the learning value of the air-fuel ratio for causing convergence of the amount of feedback correction of the air-fuel ratio calculated based on the exhaust air-fuel ratio to within a predetermined reference amount of correction is prohibited. However, to perform such control, it is necessary to precisely calculate the oil dilution rate for judging if oil dilution is occurring.
Further, an air-fuel ratio sensor gradually deteriorates along with use and sometimes changes in gain characteristics. If the gain characteristics change, the output current of the air-fuel ratio sensor becomes too large or too small for the exhaust air-fuel ratio. As a result, the exhaust air-fuel ratio is mistakenly estimated, and therefore the various types of control carried out by a control device of the internal combustion engine end up being obstructed.
Therefore, PTL 3 proposes an abnormality diagnosis system diagnosing abnormality in an air-fuel ratio sensor. In such an abnormality diagnosis system, during fuel cut control wherein the internal combustion engine stops the feed of fuel to the combustion chamber, abnormality of the air-fuel ratio sensor is diagnosed based on the value of the applied voltage of the air-fuel ratio sensor. According to PTL 2, during fuel cut control, the exhaust air-fuel ratio is constant and can be recognized, and therefore it is possible to accurately diagnose abnormality of an air-fuel ratio sensor without being influenced by fluctuations in the exhaust air-fuel ratio.
However, if oil dilution causes blowby gas containing a large amount of fuel to flow through the blowby gas passage to the intake passage, a large amount of fuel will be mixed into the air taken into a cylinder during fuel cut control. Due to this fuel, the oxygen in the exhaust gas will be consumed in the exhaust passage, in particular in the exhaust purification catalyst, and therefore the exhaust air-fuel ratio during fuel cut control will be decreased.
However, in the abnormality diagnosis system described in PTL 3, fluctuation of the exhaust air-fuel ratio during fuel cut control is not considered at all. For this reason, in this abnormality diagnosis system, if oil dilution causes the exhaust air-fuel ratio to decrease during fuel cut control, it will not be possible to accurately diagnose abnormality of the air-fuel ratio sensor. Specifically, even if the air-fuel ratio sensor is normal, if oil dilution causes the exhaust air-fuel ratio to decrease during fuel cut control, the output current of the air-fuel ratio sensor and in turn the applied voltage will decrease, and therefore the normal air-fuel ratio sensor is liable to be mistakenly diagnosed as abnormal. Alternatively, if an increase in the output current and in turn the applied voltage due to an abnormality of an air-fuel ratio sensor is cancelled out by a decrease in the output current and in turn applied voltage due to a decrease in the exhaust air-fuel ratio during fuel cut control, the abnormal air-fuel ratio sensor will be misdiagnosed as normal. Therefore, to precisely diagnose abnormality of an air-fuel ratio sensor, it is desirable to know in advance the oil dilution rate at the time of abnormality diagnosis.
Therefore, in the internal combustion engine described in PTL 4, the oil dilution rate is calculated based on the amount of feedback correction of the fuel injection amount or the learning value of the amount of feedback correction (value showing amount of lasting deviation of the fuel injection amount). Further, in the internal combustion engine described in PTL 5, the viscosity of the engine oil is directly measured by a viscosity sensor to calculate the oil dilution rate, while in the internal combustion engine described in PTL 6, the oil dilution rate is directly measured by an alcohol concentration sensor.